The present invention relates to an optical pickup apparatus, objective optical element and optical information recording reproducing apparatus by which information can be recorded and/or reproduced information compatibly for different kinds of optical disks.
In recent years, tendency of a shorter wavelength of laser beam as a light source which has been used to record and/or reproduce information for optical disks, has become a main stream. For example, a laser light source having 400-420 nm wavelength, such as a blue-violet semiconductor laser; and a blue-SHG laser which converts wavelength of an infrared semiconductor laser utilizing a second harmonic wave, have been made practical. Information of 23-27 GB can be recorded on the optical disk having a diameter of 12 cm by using these blue-violet optical sources and an objective lens having NA (Numerical aperture) which is the same as a DVD (Digital Versatile Disc). When NA is increased to 0.85, information of 23-25 GB can be recorded onto the optical disk having a diameter of 12 cm. In this specification, the optical disk and an optical-magnetic disk using a blue-violet laser light source are called “a high density optical disk”.
Hereupon, the high density optical disk using the objective optical element, has larger coma caused due to the skew of the optical disk. Therefore some high density optical disks using the objective optical element with NA of 0.85, are designed so that the protective layer has thinner thickness (which is 0.1 mm, while that of DVD is 0.6 mm) than that of DVD to reduce the comma due to the skew. On the other hand, it is sometimes considered that a product, such as an optical disk player/recorder, which is capable of only recording/reproducing information for the above high-density optical disk is worthless. Taking account of a fact that, at present, DVDs and CDs (Compact Disc), onto which various kinds of information have been recorded, are on the market, the value of the product as a high-density optical disk player/recorder is increased by, for example, enabling to appropriately record/reproduce information additionally for DVDs and CDs, which a user possesses. From these backgrounds, the optical pickup apparatus installed in the high-density optical disk player/recorder is required to be capable of appropriately recording/reproducing information not only for a high-density optical disk but also a DVD and a CD.
It can be considered, as a method by which the information can be adequately recorded/reproduced while the compatibility is maintained also to anyone of the high density optical disk and DVD and further to CD, a method to selectively switch the optical system for the high density optical disk and the optical system for DVD and CD corresponding to the recording density of the optical disk to which the information is recorded/reproduced. However, it requires a plurality of optical systems, which is disadvantageous for the size-reduction and which increases the cost.
Accordingly, in order to simplify the structure of the optical pickup apparatus and to intend the reduction of cost, it is preferable to make an optical system for the high density optical disk an optical system for DVD and CD into a common optical system, and to reduce the number of optical parts contributing the optical pickup apparatus as much as possible, even when the optical pickup apparatus has compatibility. Then, providing the common objective optical element which is arranged with facing an optical disk, is most advantageous for the simplification of the construction or cost reduction of the optical pickup apparatus. Here, in order to obtain the common objective optical element for plural kinds of optical disks which use different wavelengths for recording/reproducing information, it is requires that the objective optical system is provided with an optical path difference providing structure having a wavelength dependency for the spherical aberration, which is formed thereon.
European patent application EP-A 1304689 discloses an objective optical system which has the diffractive structure as an optical path difference providing structure and can be commonly used for the high density optical disk and the conventional DVD and CD, and also discloses an optical pickup apparatus in which this objective optical system is mounted.
However, the objective optical element for use in the optical pickup apparatus which compatibly conducts recording and/or reproducing information for three different optical disks, which is written in EP-A 1304689, has a probability that the light amount used for recording and/or reproducing information is insufficient, depending on the design specification of the optical pickup apparatus, or has a problem that the unnecessary light affects a sensor for tracking operation when the tracking operation for CD is conducted, and the accurate tracking operation is hardly conducted, which is a problem. Particularly, when using an infinite optical system for all three different kinds of optical disks, in other words, a collimated light flux enters into an objective optical element, the above problems become remarkable.